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   难度：Easy
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   <h1 class="question_title">
    661. Image Smoother
   </h1>
   <p>
    Given a 2D integer matrix M representing the gray scale of an image, you need to design a smoother to make the gray scale of each cell becomes the average gray scale (rounding down) of all the 8 surrounding cells and itself.  If a cell has less than 8 surrounding cells, then use as many as you can.
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   <p>
    <b>
     Example 1:
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   <pre>
<b>Input:</b>
[[1,1,1],
 [1,0,1],
 [1,1,1]]
<b>Output:</b>
[[0, 0, 0],
 [0, 0, 0],
 [0, 0, 0]]
<b>Explanation:</b>
For the point (0,0), (0,2), (2,0), (2,2): floor(3/4) = floor(0.75) = 0
For the point (0,1), (1,0), (1,2), (2,1): floor(5/6) = floor(0.83333333) = 0
For the point (1,1): floor(8/9) = floor(0.88888889) = 0
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   <p>
    <b>
     Note:
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    <br>
   </p>
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    <li>
     The value in the given matrix is in the range of [0, 255].
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    <li>
     The length and width of the given matrix are in the range of [1, 150].
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   <h1 class="question_title">
    661. 图片平滑器
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   <p>
    包含整数的二维矩阵 M 表示一个图片的灰度。你需要设计一个平滑器来让每一个单元的灰度成为平均灰度&nbsp;(向下舍入) ，平均灰度的计算是周围的8个单元和它本身的值求平均，如果周围的单元格不足八个，则尽可能多的利用它们。
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   <p>
    <strong>
     示例 1:
    </strong>
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   <pre>
<strong>输入:</strong>
[[1,1,1],
 [1,0,1],
 [1,1,1]]
<strong>输出:</strong>
[[0, 0, 0],
 [0, 0, 0],
 [0, 0, 0]]
<strong>解释:</strong>
对于点 (0,0), (0,2), (2,0), (2,2): 平均(3/4) = 平均(0.75) = 0
对于点 (0,1), (1,0), (1,2), (2,1): 平均(5/6) = 平均(0.83333333) = 0
对于点 (1,1): 平均(8/9) = 平均(0.88888889) = 0
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   <p>
    <strong>
     注意:
    </strong>
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   <ol>
    <li>
     给定矩阵中的整数范围为 [0, 255]。
    </li>
    <li>
     矩阵的长和宽的范围均为&nbsp;[1, 150]。
    </li>
   </ol>
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